The invention relates to a continuous casting plant for the continuous casting of a thin strip, in particular a steel strip having a thickness of below 20 mm, preferably between 1 and 12 mm, comprising a mold provided with two casting rolls, with a strip-like strand being united from two half-shells and exiting in a vertically downward direction at the nip formed by the casting rolls of said mold, wherein below the nip a deflecting-supporting means is provided for deflecting the strand emerging vertically from the mold into a roughly horizontal direction, and a method for continuous casting.
To keep the structural height and hence the costs low, the strip cast on a continuous casting plant of this kind has to be deflected from the vertical into the horizontal direction of strand discharge as gently as possible. It is also advantageous to support the strip in order to keep down the tensile load acting inside the newly solidified strip at the nip as a result of its own weight.
On the one hand, this gentle deflection must afford protection to the product. This means that excessive bending stress in the outer fiber or excessive plastic deformation in the cooling-down strip must be avoided and that sliding friction along hard, rough surfaces or pointed edges must be avoided in order to eliminate scratching or possible adhering, mainly in the region of the strip edges.
On the other hand, deflection must also involve the least possible damage to the casting process taking place upstream. Usually, a controlling device (e.g. a driver) located downstream provides for the strip extracted from the mold to be carried off without damaging the product. This driver appropriately acts by means of a position control. This means that changes in the casting speed which manifest themselves in a change of the position of the strip (loop formation increases or decreases) must be corrected by the above-mentioned driver (master-slave control principle). These corrective actions of the driver must by no means interfere with the casting operation progressing upstream, e.g. by inducing tensile stress, compressive strain or buckling strain in the hot strip just leaving the nip. Tension control is not suitable in view of the danger of rupturing the still very hot strip (low tensile strength).
To keep down sliding friction and avoid scratching or possible adhering of the strip to the deflecting-supporting means it is known to employ skids for deflection, said skids providing only linear support to the strip, which as a rule has completely solidied upon leaving of the mold, namely linear support in the longitudinal direction of the strip.
A continuous casting plant of the initially described kind is known from JP-A 63-30158. There, the strand exiting the nip of the casting-roll mold in the vertical directionxe2x80x94this narrowing is also referred to as the xe2x80x9ckissing pointxe2x80x9dxe2x80x94 is supported on both sides by a support means formed by two support means of conveyor-belt-type construction arranged parallel to each other, e.g. endless chain belts etc., and its movement is constrained over a predetermined vertical region. Subsequent to this constrainment, a guide of arcuate design extending roughly over a quarter circle is provided which serves for deflecting the strand from the vertical direction into a roughly horizontal direction.
According to JP-A 63-30158 it is difficult to ensure that deflection will damage neither the product nor the casting process, especially since the conveyor-belt-like support means arranged directly below the mold as well as subsequently arranged pinch rolls or rolls provided directly on-line exert an influence on the extraction of the strand. A further serious disadvantage is to be seen in that tension control cannot be realized between the mold and the endless chain belts on account of the danger of rupturing the strip, and on account of the danger of buckling of the strip position control cannot be realized, either. Moreover, it is not possible to ensure uniform sliding friction along the arcuate guide deflecting the strand from the vertical into the horizontal. Thus, the strand is exposed to a fluctuation of forces which influences the casting process within the mold in an unforeseeable manner and may cause disturbances during the casting operation.
Further it is known, namely from JP-A 56-119607, to provide a roller table with motor-driven rolls, in imitation of conventional continuous slab casting technology. However, this solution is disadvantageous in that the driven roller table entails high costs, especially since the rolls must not only be driven but must also be provided with internal cooling. Moreover, it is necessary that all of the rolls move synchronously with the casting rolls so as to avoid undesirable relative movements between the rollers and the strip and thus avoid any damage to the strip that might be caused thereby, and this necessitates great expenditures in terms of control engineering, an expensive drive mechanism and strong motors and hence entails additional costs. Furthermore, minor speed differentials may occur even with the most rapid response behavior of the rolls, and it is difficult to maintain the strip in a geometrically precisely defined position in order to actually achieve the optimum supporting effect.
It is further known (EP-B 0 540 610, EP-A 0 726 112 and EP-A 0 780 177) for a loop of strip which is freely suspended during the continuous casting operation to be provided between the pair of casting rolls and the first pair of pinch rolls that conveys the strip onward, resulting in the advantage that at the start of casting the size of the loop of strip will adjust automatically as a function of the casting conditions. Yet, a drawback of this method is the fact that the strand has no supporting device whatsoever; entirely unsupported, the entire weight of the strand is suspended by its hottest and hence weakest strip cross section, which is located at the nip, i.e. the kissing point. This results in a high risk of cracking or rupturing of the strip. In addition, start-up of such a plant is unfavorable, since this can only be done using a dummy bar head. To be able to start up the plant without a dummy bar head it is necessary to have start-up skids, such as those described in EP-A 0 780 177 and EP-A 0 726 112.
From U.S. Pat. No. 5,350,009 it is known in a continuous casting plant according to the preamble of claim 1 to allow the strand to get on a supporting belt moving along with the strand in the extraction direction, which belt is wound with the strand and separated again from the same later on. In order to guide the strand to the supporting belt, it is also known from that document to arrange an arcuate runner below the mold and direct the beginning of the strand toward the supporting belt. As soon as the strand is moved along with the supporting belt, the runner is placed in a resting position remote from the strand. A continuous casting plant of that type is complex in terms of construction and cumbersome to handle, the more so as a supporting belt moving along with the strand has to be provided, which must have at least the length of the continuously cast strand. That belt not only must be moved synchronously along with the strand, but it is also necessary to wind that supporting belt on and off several times in order to separate it from the strand. A continuous casting plant comprising such a supporting belt involves not only high investment costs, but also high operating costs. Furthermore, an arcuate runner is difficult to manufacture, in particular if such a runner is to be provided with a cooling means. In addition, runners of that type do not offer a large area support such that the very thin hot cast strand strip is not offered a good support, which constitutes a problem, in particular in the starting phase, in which the arcuate runners are employed according to that document.
The invention aims at avoiding the above disadvantages and difficulties and providing a continuous casting plant of the initially described kind enabling the strand which exits the mold, i.e. the as cast hot strip, to be deflected from the vertical direction into the horizontal direction while avoiding great bending stresses and avoiding major plastic deformation and while further avoiding great tensile loads.
According to the invention, this object is achieved in that the deflecting-supporting means is of plate-shaped construction and has a surface supporting the strand over a large area, preferably across its entire width.
In addition to the advantages of solving the problem underlying the invention, a plant of this kind offers the advantage that the leading end of the hot strip can be deflected and conveyed onwards as far as to the first driver, even if casting is initiated without using a cold strip, i.e., if casting is initiated without a starter bar or dummy bar. Further it is possible to cool the cast strip as a function of the quality being cast or also to prevent it from cooling down too much, for which purpose a heat-conducting surface, such as one made from copper or a copper alloy, or a heat-insulating surface, such as one made from ceramics,xe2x80x94as the case may bexe2x80x94is provided on the deflecting-supporting means.
Moreover, only a slight influence on the casting process as a result of this deflection and a careful treatment of the strand are to be ensured according to the invention and, in the case of variations in the casting speed, simple control of strand conveyance is to be feasible without interfering with the casting process in doing so. This is accomplished in that gas transit channels open into the surface of the deflecting-supporting means and are connectable to a gas conveying means.
Numerical calculations have shown that by means of the continuous casting plant of the invention the load on the strand can be alleviated by more than 40% in terms of the tensile stress acting on the strand on the site of the nip as compared to the prior art. The strand is relieved to an even greater extent when comparing a continuous casting plant according to the invention with a plant comprising a freely suspended loop, such as the one described f.i. in EP-B 0 540 610.
A strand supporting means for a horizontal continuous casting plant comprising a melt-receiving vessel provided with a melt outlet past which a casting surface can be moved so as to receive a thin layer of melt thus forming a strand, is known from AT-B 402 266, this known strand supporting means being provided with gas transit channels capable of being connected to a gas conveying means in order to reduce friction between the as yet very thin skin of the strand and the strand supporting means. In this way it is feasible to create a gas cushion between the strand and the strand supporting means such that the strand which still has a very thin skin with melt located thereon is guided on the strand supporting means in almost frictionless manner, thus preventing the occurrence of cracks or scoring etc.
According to a preferred embodiment, thermocouples are provided below the surface of the deflecting-supporting means to serve as sensors for determining the bearing site of the strand on the surface.
Another embodiment is characterized in that, laterally of the deflecting-supporting means, sensors, preferably infrared sensors, are provided for determining the bearing site of the strand on the deflecting-supporting means.
According to the invention, the deflecting-supporting means advantageously is comprised of two or several plate-shaped parts consecutively arranged in the strand extraction direction and is arranged so as to be inclined relative to the horizontal, wherein the inclination of the deflecting-supporting means or at least of a part thereof suitably lies in a range of between 10 and 60xc2x0, preferably 15 and 40xc2x0, with respect to the horizontal.
Advantageously, the deflecting-supporting means or at least a part thereof is capable of being inclined relative to the horizontal by an adjustment means.
Particularly careful deflection is achieved if the deflecting-supporting means is of concave construction on its side facing the strand, wherein the deflecting-supporting means suitably has a concave and a plane portion.
Another preferred embodiment is characterized in that the deflecting-supporting means is constructed in several parts, the parts being arranged so as to be inclined at different inclinations relative to the horizontal, and wherein suitably at least one individual part of the deflecting-supporting means is capable of being inclined relative to the horizontal by means of an adjustment means individually and independently of other parts of the deflecting-supporting means. Further, it is advantageous that the individual parts of the deflecting-supporting means be hinged to each other.
Advantageously, the gas conveying means is constructed as a means for pressurizing the gas, such as inert gas or air, that is to be conveyed through the gas transit channels. According to another suitable embodiment, the gas conveying means is constructed as a means for imparting a negative pressure on the gas that is to be conveyed through the gas transit channels. This renders it possible to maintain the strand in contact with the deflecting-supporting means during continuous operation, thus ensuring thorough cooling of the strand, especially if in accordance with another preferred embodiment the surface of the deflecting-supporting means or at least of a part thereof is made of a material of high thermal conductivity, in particular copper or a copper alloy. Preferably, this material is provided with a wear-resistant layer such as a Cr or Ni layer of an alloy or a ceramic layer.
Herein, it is suitable for the deflecting-supporting means or at least for a part thereof to be provided with an internal cooling, in particular an internal liquid cooling.
In order to avoid excessive cooling of the strand, the surface of the deflecting-supporting means or at least of a part thereof advantageously is formed of a heat-insulating material, such as ceramics.
In order to keep down gas consumption or get by with only a small-capacity gas conveying means, the gas transit channels at their mouths opening into the surface of the deflecting-supporting means suitably occupy a total cross-sectional area of 0.01 to 20%, preferably 0.1 to 5%, of the strand-supporting surface of the deflecting-supporting means, wherein, advantageously, the gas transit channels at their mouths opening into the surface of the deflecting-supporting means each have a cross-sectional area of 1 to 50 mm2, preferably 5 to 30 mm2.
The generation of a gas cushion, which is beneficial to the casting process, is ensured if the mouths of the gas transit channels are directed such that a gas stream moving substantially in the strand extraction direction is formed.
A method of operating a continuous casting plant according to claim 1 is characterized in that a predetermined pressure is adjusted between the lower surface of the strand and the deflecting-supporting means by appropriate suction and/or feeding of gas via the gas transit channels. In this way, friction between the strip and the surface of the table and hence the supporting effect can be increased in particularly hot-brittle casting operations at greater angles of inclination.
By applying suction and/or supplying a gas to all gas transit channels or only a part thereof, it is feasible to bring the neutral point which is present in the strand and on which neither tensile nor compressive forces occur as the strand is conveyed out of the mold and deflected to the horizontal into a position as close to the mold as possible, i.e. as close to its nip as possible, such that the strand will be least stressed with tensile and/or compressive forces where it is hottest.